The present invention relates to alkaline electrochemical cells that contain a conductive polymer additive for improving discharge performance. The invention is applicable to both primary and secondary cells.
The structures of alkaline electrochemical cells are well known. Typically, a cell includes an anode, a cathode, a separator between the anode and the cathode, and an electrolyte in contact with both the anode and cathode. The anode typically includes metal (often zinc) particles, and where the anode is a gelled anode, a gelling agent in the electrolyte. In the normal operation of an alkaline electrochemical cell, the metal particles must remain in electrical contact with adjacent particles and with a current collector. When particles become isolated from one another, the particles cannot participate in the electrochemical process and the discharge capacity is reduced. To maintain electrical contact between zinc particles and the current collector, a intercalating conductive carrier is typically added to the anode. In the past, mercury was employed, in the form of amalgamated zinc particles. However, as a result of environmental concerns, mercury is now substantially absent from the anodes. In its place, various conducting agents have been employed. Conducting agents are also employed in cathodes. Suitable conducting agents include polymers coated with conductive materials, and a number of such polymers have been employed.
International publication number WO 93/00716 (International Application No. PCT/CA92/00270) discloses alkaline zinc anodes for rechargeable cells in which the anode comprises zinc particles, zinc oxide, alkaline electrolyte, gelling agent and a conductive fiber structure admixed with the anode mass. The fibers themselves are described as being preferably of a non-conductive material such as polyimide having a conductive coating of copper, silver, gold or nickel. Such conductive metalized polymer fibers brought about an improvement of up to twenty percent in accumulated cycle capacity for a rechargeable C-size alkaline battery. No polymer is identified as suitable for use apart from non-conductive polymers coated with a conductive material. There is no indication of the suitability of, e.g., polyaniline, a known anticorrosive bulk electro-active polymer. See also Taucher, W. et al., “Conductive Fillers for Immobilized Alkaline Zinc Anodes,” J. of Appl. Electrochem., 22:86-98 (1992).
U.S. Pat. No. 6,174,623 describes improvements to electrodes in solid or liquid electrochemical cells where a conductive polymer such as polyaniline is added to an electrode to improve conductivity. It is also known in the art to provide anode and cathode electrodes made of a major amount of a conducting polymer as the electrode itself. U.S. Pat. No. 6,174,623 notes that many conducting polymers are difficult to work with and some are simply intractable high molecular weight materials, insoluble in ordinary solvents and prone to decomposition below their melting or softening point. To avoid these issues, the patent, which relates primarily to solid rather than liquid electrochemical cells, requires treatment of the conducting polymer to render it suitable for coating the active material. The treatment steps include dissolving the conducting polymer in a solvent and mixing the solubilized conducting polymer with the active particles and then removing substantially all of the solvent from the mixture to form a paste suitable for curing to form a solid electrode. Such processing steps have effects on the electrical conductivity of the polymer. For example, solution casting of film coatings onto substrates can significantly impact the polymer chain conformations as gelation occurs during solvent evaporation. Additionally, the choice of solvent can determine whether the polymer chain exhibits an expanded or compacted structure. Both chain confirmation and morphology can significantly affect the π-conjugation lengths and electronic levels of electronic conducting polymers such as polyaniline which are composed of conjugated polymer chains with π electrons delocalized along the backbone.
U.S. Pat. No. 5,645,890 discloses methods for inhibiting surface corrosion on metal substrates by depositing a polyaniline coating onto an exposed surface. The surface is contacted by a solution that contains the polyaniline dissolved in an organic solvent.
A negative impact on the electronic conductive properties of polyaniline is known, when the polyaniline is dissolved in an organic solvent and used at high pH. For example, Alonso et al., J. Electroanal. Chem., 481:200-207 (2000) studied the catalytic behavior of a molybdenum-doped ruthenium selenide catalyst supported on a polyaniline matrix. In that work, polyaniline was synthesized and converted to emeraldine base before being dispersed in an organic solvent with the catalyst. The solvent was evaporated from the resulting dispersion to prepare the catalyst supported on the polyaniline matrix. The catalytic oxygen-reducing activity of these electrodes was higher at a pH of up to 3, where the dispersed catalyst shows good catalytic activity without impact on the conductivity of the polyaniline matrix. At higher pH, however, catalytic activity was lower due to the loss of polyaniline conductivity. Thus, it is unacceptable to include an electronic conducting polymer dissolved in an organic solvent in a process for preparing an electrode for use in an alkaline environment. The use of such dissolved polymers is also unacceptable in a process for manufacturing a battery as it introduces solvent residues and impurities having unknown effects on electrochemical cells.
However, it is an important goal of the battery industry to develop suitable replacements for environmentally unacceptable intercalation compounds where the replacements are compatible with existing manufacturing processes and performance requirements and have no detrimental effect upon discharge performance. There is therefore a need to develop an effective process for employing a conductive polymer additive in an alkaline electrochemical system, where the process is free of organic solvent steps and requires no curing step.